The present invention relates generally to Magnetic Resonance Imaging (MRI) systems, and more particularly, to a method and system for simulating MRI magnet field instability due to mechanical disturbances.
Currently, Magnetic Resonance Imaging (MRI) systems have included a superconducting magnet that generates a temporally constant primary magnetic field. The superconducting magnet must keep the coils at approximately 4K for low temperature superconductors. The superconducting magnet is contained within a helium vessel having a radiation shield assembly. The vessel is referred to as a cryostat. A cryocooler maintains the superconducting magnet at a temperature of 4.2K.
The superconducting magnet is used in conjunction with a magnetic gradient coil assembly, which is sequentially pulsed to create a sequence of controlled gradients in the main magnetic field during a MRI data gathering sequence. The superconducting magnet and the magnetic gradient coil assembly have a radio frequency (RF) coil on an inner circumferential side of the magnetic gradient coil assembly. The controlled sequential gradients are effectuated throughout a patient imaging volume (patient bore) which is coupled to at least one MRI (RF) coil or antennae. The RF coils and a RF shield are typically located between the magnetic gradient coil assembly and the patient bore.
As a part of a typical MRI, RF signals of suitable frequencies are transmitted into the patient bore. Nuclear magnetic resonance (nMR) responsive RF signals are received from the patient via the RF coils. Information encoded within the frequency and phase parameters of the received RF signals, by the use of a RF circuit, is processed to form visual images. These visual images represent the distribution of nMR nuclei within a cross-section or volume of the patient within the patient bore.
Mechanical disturbances within the MRI system create eddy currents causing electro-magnetic field instability within the MRI system. The electro-magnetic field instability detrimentally effects image quality. To maintain high quality images during operating conditions, the superconducting magnet requires very high main field stability in order to minimize ghosting or false images.
The mechanical disturbances are due to both internally and externally caused mechanical movement of MRI system components. Examples of some internal mechanical disturbances are cryostat vibration, superconducting magnet movement, and gradient coil movement due to induced impulses. The cryostat is coupled to a pump which, when operating, causes the cryostat to vibrate or generate mechanical impulses, disturbing the main field. Both the superconducting magnet and the magnetic gradient coil assembly are impulsed during operation causing magnetic coils positions to be altered and mechanical movement to exist within the main magnetic field. A one micro-inch motion of the magnet can produce sufficient field instability to cause a ghosting problem. An example of an externally caused mechanical disturbance is environmental movement from human activity around the MRI system causing floor vibrations to transfer into the MRI system.
It would therefore be desirable to design the superconducting magnet, supporting structures, and related components so as to minimize mechanical disturbances, thereby, minimizing induced main field disturbances and ensuring and potentially increasing image quality.
The present invention provides a method and apparatus for simulating MRI magnet field instability due to mechanical disturbance. A Magnetic Resonance Imaging (MRI) magnet field instability simulator is provided. The simulator includes a rigid body motion generator that simulates motion of one or more MRI system components. An eddy current analyzer generates a magnetic stiffness and damping signal and an electromagnetic transfer function in response to the motions and a cryostat material properties signal. A mechanical model generator generates a mechanical disturbance signal and a mechanical model of one or more MRI system components in response to the motions and the magnetic stiffness and damping signal. A structural analyzer generates a motion signal in response to the mechanical model. A field instability calculator generates a field instability signal in response to the electromagnetic transfer function and the motion signal. A method of performing the same is also provided.
One of several advantages of the present invention is the ability to simulate effects of MRI main field disturbances within a MRI system. The simulation of these effects provides a design tool for better and more efficient engineering of MRI system components. Thus, providing a method of mimicking a resulting MRI system without the traditional time and expense involved in actual construction of the MRI system or system component.
Another advantage of the present invention is the ability to adjust MRI system component design within a relatively short period of time, along with quickly viewing the resulting effects of the alteration. Design by simulation trial and error, which was not normally practical for full MRI systems because of the large expense involved, is now feasible.
Furthermore, the present invention provides the ability to perform detailed analysis of MRI system components and relate the results of this analysis to mechanical disturbances and generated eddy currents.
Moreover, the present invention in simulation of field disturbances accounts for not only MRI system internally caused field disturbances but also externally caused field disturbances, allowing for design of externally related MRI system components.
The present invention itself, together with attendant advantages, will be best understood by reference to the following detailed description, taken in conjunction with the accompanying figures.